It's extremely unusual - if not totally unheard of - for a new Off-Broadway show to have two consecutive opening nights.

But that proved to be necessary for "An Iliad," a mesmerizing one-man riff on Homer's 15,000-line epic poem being performed by two different Tony-winning actors at alternating performances.

The 100-minute monologue, co-written by Lisa Peterson and actor Denis O'Hare, was originally set to be performed by O'Hare last year at New Jersey's McCarter Theatre. But when a conflict arose for O'Hare, Stephen Spinella took his place. For the Off-Broadway premiere, both actors got involved.

Based loosely on Robert Fagles' celebrated translation's, "An Iliad" revolves around an anonymous, traveling storyteller who, on an empty stage, speaks directly to the audience and recalls parts of the Trojan War, bringing the narrative back to its roots as a product of oral storytelling.

The Poet, as he is called in the program, begs us to think of the Greek men sailing toward Troy as if they were teenagers from different parts of the United States and to wonder what it's like to be away from home for nine years.

After dramatizing selections of the bloody, chaotic exploits of Hector, Achilles and Agamemnon, the Poet rattles off a long list of all the world wars that have taken place since, emphasizing the authors' belief that war is a wasteful, never-ending enterprise.

Backed by Mark Bennett's harsh sound design, Brian Ellingsen's haunting music and Scott Zielinki's shadowy lighting, "An Iliad" makes for a gripping theatrical experience whether you see it with Spinella or O'Hare. But there are key differences between their two performances.